   * {padding:0;margin:0}
    html,body {
        height: 100%;
        background-color: black;
    }
    .xuetiao {
    position: fixed;
    left: 0px;
    top: 0px;
    width: 100px;
    height: 50px;
    border: solid 5px yellow;
}
    .xuetiao span {
    width: 100px;
    height: 50px;
    background-color: red;
    display: block;
}